The present invention relates to a light beam scanner such as a laser printer for scanning a recording surface by a laser to record a picture image thereon.
FIG. 2 is a perspective view of an optical system showing one example of a conventional laser printer in which a light source 1 such as a semiconductor laser, etc., emits a laser beam as a light beam. A rotary polygonal mirror 2 is used as a scanning section, and a photosensitive drum 3 has a recording surface with photodetector 4 arranged in the vicinity thereof.
The laser beam emitted from light source 1 is reflected by rotary polygonal mirror 2, and is irradiated onto photosensitive drum 3. The scanning operation is performed by the laser beam in a main scanning direction parallel to the rotary axis of photosensitive drum 3 according to the rotation of rotary polygonal mirror 2. Rotary photosensitive drum 3 is also rotated in an auxiliary scanning direction around the rotary axis thereof. Thus, a predetermined image is recorded onto photosensitive drum 3 by the laser beam which is on/off-modulated corresponding to a recording signal.
Photodetector 4 is arranged in a predetermined position on the scanning plane of the laser beam. The scanning position (i.e., the rotary position of rotary polygonal mirror 2) of the laser beam is controlled with the position of photodetector 4 as a reference, thereby providing an image without shift.
However, in the conventional light beam scanner as mentioned above, the scanning position of the laser beam is detected by photodetector 4. As a result, when a reflecting face of rotary polygonal mirror 2 is inclined from its regular position, the laser beam is not incident on photodetector 4. Consequently, it becomes necessary to adjust the attaching position of photodetector 4, which is inconvenient and time-consuming. The impact of the irregular inclination of the reflecting face of rotary polygonal mirror 2 can be reduced by making photodetector 4 large-sized, for example, but the cost of the scanner is increased by such a construction.
To solve the above-mentioned problems, an object of the present invention is to provide a light beam scanner in which the cost of the scanner is not increased substantially over conventional scanners, and yet, in which it is unnecessary to adjust the position of the photodetector 4 in order to make the laser beam incident thereto.